Powering the iPad for four hours requires 4.24 amp-hours of battery capacity (1.06 A × 4 hrs.). (The table values already include a 15% inverter efficiency loss.) To meet this daily electrical load, let’s assume you have six hours of bright sun to recharge the battery. This means you will need a PV module that can provide a minimum of 0.71 A output (4.24 Ah ÷ 6 hrs.). However, to account for the miscellaneous efficiency losses during this charging process, I have added a 15% loss factor, which rounds this up to 0.8 A. Since the nominal 17 V PV module voltage will vary between 14 and 17 V as the battery voltage starts to rise, this means that the PV module will need to be rated at a minimum of 14 W (0.8 A x 17 V). For small electronic devices, it is always more efficient to use a 12 VDC charger, if available, to avoid inverter efficiency losses. A PV module twice as large will cut the charge time in half—from six to three hours—so keep this in mind when comparing systems with different wattage modules.

Watching Watts

Selecting a portable PV system can be confusing, since there are no standards for rating this type of equipment. In addition, some people just do not understand how much electrical energy an appliance consumes when it operates, since their only experience with electricity is plugging into a wall outlet—a practically unlimited amount of energy.

Some marketing hype claims that their products can power all kinds of household appliances, including refrigerators, microwave ovens, desktop computers, and household lighting. Although it may be theoretically possible to power these large loads if the inverter is large enough, the actual battery capacity may be for minutes—not hours or days. To keep production costs low, some manufacturers may provide a very small PV module to recharge their higher-capacity battery, which means it may take days to recharge the battery after it powers a large load for only a few minutes.

When trying to sort out all these advertising claims, you will find few terms in common. Some product advertising includes helpful amp-hour ratings for battery capacity, others may provide battery capacity in “joules,” or worse, may not provide any battery capacity information at all.

That large “joule” rating advertised for one system’s battery seems hundreds of times higher than the watt-hour ratings listed for the competition. However, a joule is just a “watt-second,” so a battery that provides 30 watt-hours of standby power would have a rating of 108,000 joules (30 Wh × 60 min./hr × 60 sec./min.). The same 30 Wh rating for a 12 V battery equals 2.5 Ah if totally discharged, but only 1.25 Ah if discharged to a more realistic 50% rate, so you can see how this can be very misleading.

Regardless of which portable solar power system you need, there are now many different sizes to pick from. The recent drop in the cost of PV modules is making them even more affordable, and newer battery types are helping to keep the weight down.

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Jeff Yago (pvforyou.com) is an engineer and certified energy manager in Virginia, and has more than 35 years of experience in the solar and emergency preparedness field. He is a NABCEP-certified PV installer, and has authored numerous articles and texts.